The term “programmable patterning structure” as here employed should be broadly interpreted as referring to any programmable structure or field that may be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of a substrate; the term “light valve” can also be used in this context. Generally, such a pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such programmable patterning structure include:
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of very small (possibly microscopic) mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation structure. For example, the mirrors may be matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic structure. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which documents are herein incorporated by reference, and PCT patent applications WO 98/38597 and WO 98/33096, which documents are herein incorporated by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which document is herein incorporated by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
An imaging apparatus is currently employed to make mask writing machines (e.g. by the Swedish firm Micronic). The substrate in such a mask writing machine is, for example, a metallized plate (e.g. a Cr-coated quartz or CaF2 plate) that has been coated with a layer of photoresist. The idea behind such a mask writing machine is that an electronic file of the mask pattern (which pattern is typically highly complex) is used to matrix-address the patterning structure, which then diverts a patterned radiation beam onto a small portion of the mask plate. By changing the pattern in the patterned beam in accordance with the electronic file, and concurrently moving the beam over the whole surface of the mask plate (in either a scanning or a stepping motion), the final mask pattern is built up as a sum of combined juxtaposed) sub-patterns from the patterned beam. For this reason, such a machine is sometimes referred to as a “mask writer”. A mask as produced by such an apparatus can be used in a lithographic projection apparatus, which repetitively images the mask pattern onto a photo-sensitive substrate—such as a photoresist-coated semiconductor (e.g. Si, Ge, GaAs, SiGe) wafer—as part of the broader manufacturing process involved in producing integrated devices, such as integrated circuits (ICs).
One factor limiting a wider use of mask writing techniques in lithographic projection practices (e.g. for direct writing to a substrate) is the very low throughput: whereas current direct-write machines might be expected to achieve a throughput on the order of one substrate per day, a state-of-the-art lithographic projection apparatus has a throughput of the order of 100 substrates per hour. Therefore, use of mask writing techniques for direct writing to a substrate is currently limited to those cases in which the cost of enduring a long writing process for each wafer is less than the cost of preparing a special mask for a low-quantity production run.